In digital communications, a carrier with modulation implemented either in amplitude, frequency, or phase can be decomposed into, or synthesized from, two band pass modulated sinusoids that are offset in phase by one-quarter cycle (π/2 radians). The orthogonal basis functions used to implement modulation or demodulation are known as in-phase and quadrature components.
Consider the two axes of the Cartesian plane: an in-phase axis (x-axis) that is a zero degree axis and the quadrature axis (y-axis) that is a ninety degree axis (the phase plane). The input signal, which is an input vector, may occur anywhere in that plane. It can rotate anywhere in the (x,y) plane. It can occur in any of the four quadrants of that phase plane. The vector may be represented as a combination of some components in the x-axis and the y-axis so that the sum of the squares of the individual projections equals to the square of the magnitude that is contained in the vector—it is a vector in the X and Y components, or a Cartesian representation of a vector. The quadrature component may be used in digital communication systems to perform image rejection and improve data density.
One example quadrature component generator, as shown in FIG. 1A, uses voltage controlled oscillator (VCO) 105, buffer 110, and divide by two network 115, which may be, for example, two D latches connected in a positive feedback loop. This generates an in-phase and a quadrature wave forms at half the VCO frequency, which is a 90 degree phase shifted version of the in-phase component. The output of divide by two network 115 is filtered with LC tuned filter 120 and then provided to mixer 125. In that scheme, quadrature generation is accurate but the VCO operates at twice the output frequency. This leads to the divider consuming high current, which may be prohibitive as the frequencies increase.
In another example, as shown in FIG. 1B, LC-tuned filter 135 filters the output of VCO 130 to reject the harmonics and produce a pure sinusoid signal. The sinusoid signal is then applied to poly-phase IQ generator 140, which comprises, for example, a network of resistors and capacitors to produce a phase lead and a phase lag. This network creates a phase lead of 45 degrees and a phase lag of 45 degrees, which results in a total phase difference of 90 degrees between the two output phases. The resulting signal is again filtered with LC-tuned filter 145 and then on to mixer 150 for functional uses. A disadvantage of the signal generation scheme is that quadrature generation using passive polyphase phase shifter 140 provides large signal loss, so a buffer may be used after poly-phase filter 140 to compensate for the signal loss. This buffer at the output of poly-phase filter 140 consumes additional current and contributes noise.
Another scheme, as shown in FIG. 1C, uses two cross coupled oscillators, VCOI 160 and VCOQ 170. The current consumption is comparatively high. In this scheme, the phase noise adds up uncorrelated to produce high spectral content. Quadrature phase generation with harmonic filtering of the reference oscillation signal is fundamental to any wireless transceiver that provides moderate-to-high selectivity. As shown above, conventional RF techniques involve oscillating at twice the required frequency followed by a divide by two block for IQ generation. Previous techniques using transmission line based hybrid I/Q generation techniques are limited due to being bulky and area inefficient, lossy and power inefficient, limited in phase shift and quadrature accuracy, and limited in rejection of higher order harmonics. There are heretofore unaddressed needs with these previous solutions.